Thin film transistor substrate and manufacturing method thereof

ABSTRACT

A thin film transistor substrate includes: a substrate; an oxide semiconductor layer disposed on the substrate; a gate electrode disposed on the substrate; a gate insulating layer interposed between the oxide semiconductor layer and the gate electrode; and a source electrode and a drain electrode connected to the oxide semiconductor layer, the source electrode and the drain electrode being spaced apart from each other. The gate insulating layer includes: a first gate insulating layer having an oxygen content lower than that of a stoichiometric composition; and a second gate insulating layer including a material substantially the same as a material which the first gate insulating layer may include, and having an oxygen content higher than that of the first gate insulating layer, and the first gate insulating layer and the oxide semiconductor layer directly contact each other.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0051982, filed on May 4, 2018, in the KoreanIntellectual Property Office (KIPO), the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND 1. Field

Embodiments of the present invention relate to a thin film transistor(“TFT”) substrate and a method of manufacturing the TFT substrate, andmore particularly, to a TFT substrate utilizing a metal oxidesemiconductor thin film and a method of manufacturing the TFT substrate.

2. Description of the Related Art

In general, TFTs are utilized in various electronic devices, such asflat panel display devices. For example, the TFTs are utilized asswitching elements or driving elements in various flat panel displaydevices, such as liquid crystal display (“LCD”) devices, organic lightemitting diode (“OLED”) display devices, and electrophoretic displaydevices.

Such a TFT includes a gate electrode connected to a gate line fortransmitting a scan signal, a source electrode connected to a data linefor transmitting a signal to be applied to a pixel electrode, a drainelectrode facing the source electrode, and a semiconductor layerelectrically connected to the source and drain electrodes.

The semiconductor layer of the TFT is an important element fordetermining the characteristics of the TFT. Such a semiconductor layergenerally includes amorphous silicon, polycrystalline silicon or oxidesemiconductor. Metal oxides, however, have higher electron mobility thanthat of amorphous silicon, lower cost than that of polycrystallinesilicon, and higher uniformity than that of polycrystalline silicon.

It is to be understood that this background of the technology section isintended to provide useful background for understanding the technologyand as such, the technology background section may include ideas,concepts or recognitions that were not part of what was known orappreciated by those skilled in the pertinent art prior to acorresponding effective filing date of subject matter disclosed herein.

SUMMARY

In order to apply the metal oxide semiconductor to the TFT, the chargemobility and the reliability should be increased. However, when such ametal oxide semiconductor layer is formed, oxygen defects may largelyoccur due to, for example, displacement of excess oxygen, and thus thefilm quality of the semiconductor layer may be lowered. Accordingly, theelectrical characteristics and the photoelectric reliability ofelements, e.g., a threshold voltage Vth of the TFT, may be degraded.

Aspects according to embodiments of the present invention may bedirected to a TFT substrate that has high reliability by significantlyreducing oxygen defects in an oxide semiconductor and at an interfacebetween the oxide semiconductor and a gate insulating layer, and to amethod of manufacturing the TFT substrate.

According to an embodiment, a thin film transistor substrate includes: asubstrate; an oxide semiconductor layer on the substrate; a gateelectrode on the substrate; a gate insulating layer between the oxidesemiconductor layer and the gate electrode; and a source electrode and adrain electrode both connected to the oxide semiconductor layer, thesource electrode and the drain electrode being spaced apart from eachother. The gate insulating layer includes: a first gate insulating layerincluding a first material and having an oxygen content lower than thatof a stoichiometric composition of the first material; and a second gateinsulating layer including a second material substantially the same asthe first material, and having an oxygen content higher than that of thefirst gate insulating layer, and the first gate insulating layer and theoxide semiconductor layer directly contact each other.

The first gate insulating layer may include a metal oxide or a metaloxynitride that has an oxygen content lower than that of thestoichiometric composition of the first material.

The first gate insulating layer may include one or more materialsselected from: SiO_(x) (0<x<2), AlO_(x) (0<x<3/2), ZnO_(x) (0<x<1),ZrO_(x) (0<x<2), HfO_(x) (0<x<2), GaO_(x)(0<x<3/2), TiO_(x) (0<x<2),TaO_(x) (0<x<3/2), MnO_(x) (0<x<2), LaO_(x) (0<x<3/2), SiO_(x)Ny (0<x<1,0<y<2/3, and 0<x+y<5/3), AlO_(x)Ny (0<x<3/4, 0<y<1/2, and 0<x+y<5/4),and GaO_(x)Ny (0<x<3/4, 0<y<1/2, and 0<x+y<5/4).

The first gate insulating layer may include silicon oxide represented bySiO_(x), where 0<x<2, and an atomic weight ratio O/Si of oxygen tosilicon in the silicon oxide may be substantially equal to or more thanabout 1.5 and less than about 2.0.

The silicon oxide may have an atomic weight ratio of O/Si that decreasesalong a thickness direction as a distance from the oxide semiconductorlayer decreases.

The first gate insulating layer may have an excess hydrogen content ofabout 5.0×10²⁰ atoms/cm³.

The first gate insulating layer may have a hydrogen content that variesalong a thickness direction of the first gate insulating layer.

The first gate insulating layer may have a thickness substantially equalto or less than about 500 Å.

The gate insulating layer may further include a third gate insulatinglayer including a third material different from the first material.

The third gate insulating layer may is an oxide-based insulating layer,an oxynitride-based insulating layer, or a nitride-based insulatinglayer.

The oxide semiconductor layer may include a metal oxide semiconductorincluding oxygen, and has a density of oxygen defects of about 2.0×10¹⁷cm⁻³ eV⁻¹.

The oxide semiconductor layer may include at least one selected from:indium (In), gallium (Ga), tin (Sn), zirconium (Zr), vanadium (V),hafnium (Hf), cadmium (Cd), germanium (Ge), chromium (Cr), titanium(Ti), and zinc (Zn).

The thin film transistor substrate may include: the substrate; the oxidesemiconductor layer on the substrate; the first gate insulating layer onthe oxide semiconductor layer; the second gate insulating layer on thefirst gate insulating layer; the gate electrode on the second gateinsulating layer; and the source electrode and the drain electrode bothconnected to the oxide semiconductor layer, the source electrode and thedrain electrode being spaced apart from each other.

The thin film transistor substrate may include: the substrate; the gateelectrode on the substrate; the second gate insulating layer on the gateelectrode; the first gate insulating layer on the second gate insulatinglayer; the oxide semiconductor layer on the first gate insulating layer;and the source electrode and the drain electrode connected to the oxidesemiconductor layer, the source electrode and the drain electrode beingspaced apart from each other.

According to an embodiment, a method of manufacturing a thin filmtransistor substrate includes: forming an oxide semiconductor layer on asubstrate; forming a gate insulating layer on the oxide semiconductorlayer, the gate insulating layer including at least two layers; forminga gate electrode on the gate insulating layer; and forming a sourceelectrode and a drain electrode on the gate insulating layer andconnecting each of the source electrode and the drain electrode to theoxide semiconductor layer. The forming of the gate insulating layerincludes: (i) depositing a first gate insulating layer on the oxidesemiconductor layer utilizing at least one of a first power (P1) and afirst volume ratio of a reaction gas to a metal precursor gas (G1), thefirst gate insulating layer including a first material and having anoxygen content lower than that of a stoichiometric composition of thefirst material; and (ii) depositing a second gate insulating layerincluding a second material substantially the same as the first materialutilizing at least one of a second power (P2) and a second volume ratioof a reaction gas to a metal precursor gas (G2), the second gateinsulating layer having an oxygen content higher than that of the firstgate insulating layer, where P1<P2 and G1<G2.

The the (i) the depositing of the first gate insulating layer and (ii)the depositing of the second gate insulating layer may be performedunder a condition that satisfies the following Equation 1:

$\begin{matrix}{{\frac{P_{1}}{P_{2}} \leq 0.8},} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

where P1 is the first power utilized in depositing the first gateinsulating layer, and P2 is the second power utilized in depositing thesecond gate insulating layer.

The the (i) the depositing of the first gate insulating layer and (ii)the depositing of the second gate insulating layer may be performedunder a condition that satisfies the following Equation 2:1≤G1<G2,  Equation 2

where G1 i the volume ratio of the reaction gas to the metal precursorgas when depositing the first gate insulating layer, and G2 is thevolume ratio of the reaction gas to the metal precursor gas whendepositing the second gate insulating layer.

The reaction gas utilized in the depositing of the first or second gateinsulating layer may be one or more gases selected from N₂O, O₃, O₂,NH₃, H₂, and N₂, and the metal precursor gas utilized in the depositingof the first or second gate insulating layer may be a precursor gasincluding at least one selected from Si, Al, Ti, Ga, Ta, Mn, Zn, Zr, Hfand La.

The forming of the gate insulating layer may further include: (iii)forming, on the second gate insulating layer, a third gate insulatinglayer including a third material different from the first material.

The method may further include performing annealing after the forming ofthe gate insulating layer.

The foregoing is illustrative only and is not intended to be in any waylimiting. In addition to the illustrative aspects, embodiments andfeatures described above, further aspects, embodiments and features willbecome apparent by reference to the drawings and the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention will become moreapparent by describing in more detail embodiments thereof with referenceto the accompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating a TFT substrate accordingto an embodiment of the present invention;

FIG. 2 is a cross-sectional view illustrating a TFT substrate accordingto another embodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a TFT substrate accordingto another embodiment of the present invention;

FIGS. 4 to 8 are cross-sectional views schematically illustrating amethod of manufacturing the TFT substrate of FIG. 1;

FIG. 9 is a graph illustrating changes in plasma power utilized formanufacturing the TFT substrate of FIG. 1;

FIG. 10 is a graph illustrating changes in gas composition utilized formanufacturing the TFT substrate of FIG. 1;

FIG. 11 is a secondary ion mass spectroscopy (“SIMS”) graph illustratingchanges in the content of hydrogen in a first gate insulating layeraccording to the plasma power;

FIG. 12 is a SIMS graph illustrating changes in the content of hydrogenincluded in a gate insulating layer according to the sub-layers;

FIG. 13 is a graph illustrating an O/Si atomic weight ratio in the gateinsulating layer;

FIG. 14 is a graph illustrating changes in density of state (“DOS”) of aconduction band of an oxide semiconductor layer;

FIG. 15 is a graph illustrating changes in DOS of a conduction band ofan oxide semiconductor layer;

FIG. 16 is a graph illustrating changes in DOS of a conduction band ofan oxide semiconductor layer;

FIG. 17 is a graph illustrating a positive bias temperature stress(“PBTS”) value of a TFT that does not include the first gate insulatinglayer;

FIG. 18 is a graph illustrating a PBTS value of a TFT that includes thefirst gate insulating layer; and

FIG. 19 is a graph illustrating changes in a threshold voltage Vth withrespect to a cumulative time based on the results of FIGS. 17 and 18.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with referenceto the accompanying drawings. Although the invention may be modified invarious manners and have several embodiments, example embodiments areillustrated in the accompanying drawings and will be mainly described inthe specification. However, the scope of the invention is not limited tothese embodiments and should be construed as including all the changes,equivalents and substitutions included in the spirit and scope of theinvention.

In the drawings, thicknesses of a plurality of layers and areas areillustrated in an enlarged manner for clarity and ease of descriptionthereof. When a layer, area, or plate is referred to as being “on”another layer, area, or plate, it may be directly on the other layer,area, or plate, or intervening layers, areas, or plates may be presenttherebetween. Conversely, when a layer, area, or plate is referred to asbeing “directly on” another layer, area, or plate, intervening layers,areas, or plates may be absent therebetween. Further, when a layer,area, or plate is referred to as being “below” another layer, area, orplate, it may be directly below the other layer, area, or plate, orintervening layers, areas, or plates may be present therebetween.Conversely, when a layer, area, or plate is referred to as being“directly below” another layer, area, or plate, intervening layers,areas, or plates may be absent therebetween.

The spatially relative terms “below”, “beneath”, “lower”, “above”,“upper” or the like, may be utilized herein for ease of description todescribe the relations between one element or component and anotherelement or component as illustrated in the drawings. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation, in addition tothe orientation depicted in the drawings. For example, in the case wherea device illustrated in the drawing is turned over, the devicepositioned “below” or “beneath” another device may be placed “above”another device. Accordingly, the illustrative term “below” may includeboth the lower and upper positions. The device may also be oriented inthe other direction and thus the spatially relative terms may beinterpreted differently depending on the orientations.

In addition, throughout the specification, when an element is referredto as being viewed “in a plan view”, the element is viewed from theabove, and when an element is referred to as being viewed “on across-sectional view”, a cross-sectional surface of the element that iscut vertically is viewed from the lateral side.

Throughout the specification, when an element is referred to as being“connected” to another element, the element may be “directly connected”to the other element, or “electrically connected” to the other elementwith one or more intervening elements interposed therebetween. It willbe further understood that the terms “comprises,” “comprising,”“includes” and/or “including,” when used in this specification, specifythe presence of stated features, integers, steps, operations, elementsand/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components and/or groups thereof.

It will be understood that, although the terms “first,” “second,”“third,” or the like may be used herein to describe various elements,these elements should not be limited by these terms. These terms areonly used to distinguish one element from another element. Thus, “afirst element” discussed below could be termed “a second element” or “athird element,” and “a second element” and “a third element” may betermed likewise without departing from the teachings herein. Expressionssuch as “at least one of,” when preceding a list of elements, modify theentire list of elements and do not modify the individual elements of thelist. Further, the use of “may” when describing embodiments of thepresent invention refers to “one or more embodiments of the presentinvention.”

The term “about,” “substantially” or “approximately” as used herein isinclusive of the stated value and refers to within an acceptable rangeof deviation for the particular value as determined by one of ordinaryskill in the art, considering the measurement in question and the errorassociated with measurement of the particular quantity (i.e., thelimitations of the measurement system). For example, “about” may referto within one or more standard deviations, or within +30%, 20%, 10%, 5%of the stated value. Also, any numerical range recited herein isintended to include all sub-ranges of the same numerical precisionsubsumed within the recited range. For example, a range of “1.0 to 10.0”is intended to include all subranges between (and including) the recitedminimum value of 1.0 and the recited maximum value of 10.0, that is,having a minimum value equal to or greater than 1.0 and a maximum valueequal to or less than 10.0, such as, for example, 2.4 to 7.6. Anymaximum numerical limitation recited herein is intended to include alllower numerical limitations subsumed therein and any minimum numericallimitation recited in this specification is intended to include allhigher numerical limitations subsumed therein. Accordingly, Applicantreserves the right to amend this specification, including the claims, toexpressly recite any sub-range subsumed within the ranges expresslyrecited herein.

Unless otherwise defined, all terms used herein (including technical andscientific terms) have the same meaning as commonly understood by thoseskilled in the art to which this invention pertains. It will be furtherunderstood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an ideal or excessively formal sense unlessclearly defined in the present specification.

Some of the parts which are not associated with the description may notbe provided in order to specifically describe embodiments of the presentinvention. Like reference numerals refer to like elements throughout thespecification.

Hereinafter, an embodiment of the present invention will be describedwith reference to FIGS. 1 to 3.

FIG. 1 is a cross-sectional view illustrating a TFT substrate 100according to an embodiment of the present invention.

Referring to FIG. 1, the TFT substrate 100 according to an embodiment ofthe present invention includes a TFT that includes an oxidesemiconductor as an active layer on a substrate 110. In one embodiment,the TFT substrate 100 includes the substrate 110, an oxide semiconductorlayer 120, a gate insulating layer 130, a gate electrode 150, a sourceelectrode 140 a, and a drain electrode 140 b.

The substrate 110 may include various suitable materials. For example,the substrate 110 may include a transparent glass material that includessilicon oxide (SiO₂) as a main component. However, embodiments are notlimited thereto. For example, in an exemplary embodiment of the presentinvention, the substrate 110 may include a transparent plastic materialand have flexibility. Such a plastic material may be an organic materialselected from, for example, polyethersulphone (PES), polyacrylate (PAR),polyetherimide (PEI), polyethylene naphthalate (PEN), polyethyleneterephthalate (PET), polyphenylene sulfide (PPS), polyallylate,polyimide, polycarbonate (PC), cellulose triacetate (TAC), and celluloseacetate propionate (CAP). It is not necessary that the substrate 110invariably includes a transparent material, and the substrate 110 mayinclude a suitable metal material (e.g., a metal material known in theart). For example, the substrate 110 may include at least one of, forexample, iron (Fe), chromium (Cr), manganese (Mn), nickel (Ni), titanium(Ti), molybdenum (Mo), stainless steel (SUS), invar alloys, inconelalloys, and kovar alloys. The substrate 110 is not particularly limitedto the shape, structure, size, and the like, provided that it has a mainsurface capable of forming a film thereon, and can be appropriatelyselected according to the purpose of use. The substrate 110 may have asingle-layer structure or a multilayer structure in which two or morelayers are stacked.

A buffer layer may be disposed on the substrate 110. The buffer layermay provide a planar surface on top of the substrate 110 and may blockforeign matter or moisture from penetrating through the substrate 110.For example, the buffer layer may include an inorganic material (such assilicon oxide (SiO₂), silicon nitride (Si₃N₄), silicon oxynitride(SiON), aluminum oxide (Al₂O₃), aluminum nitride (AlN), titanium oxide(TiO₂), and titanium nitride (TiN)), or an organic material (such aspolyimide, polyester, and acryl), and may have a multilayer structureincluding the above-described materials.

The TFT that includes an oxide semiconductor as an active layer isdisposed on the substrate 110. The TFT may include the oxidesemiconductor layer 120, the gate electrode 150, the source electrode140 a, and the drain electrode 140 b.

The TFT may be classified into a bottom gate type (e.g., a bottom gateTFT) and a top gate type (e.g., a top gate TFT) according to theposition of the gate electrode 150 with respect to the semiconductorlayer 120, or may be classified into a coplanar type (e.g., a coplanarTFT), a staggered type (e.g., a staggered TFT), and the like, accordingto the position of a channel and the arrangement of the source and drainelectrodes 140 a and 140 b. Hereinafter, the present embodiment will bedescribed on the premise that the TFT is a top gate type in which theoxide semiconductor layer 120, the gate electrode 150, the sourceelectrode 140 a and the drain electrode 140 b are sequentially arranged.However, the present embodiment is not limited thereto, and variouskinds of TFTs may be applied.

The oxide semiconductor layer 120 is disposed on the buffer layer. Theoxide semiconductor layer 120 may include, without limitation, asuitable oxide semiconductor material (e.g., an oxide semiconductormaterial known in the art). For example, the oxide semiconductor layermay be a metal (or metalloid) oxide including at least one elementselected from, for example, indium (In), gallium (Ga), tin (Sn),zirconium (Zr), vanadium (V), hafnium (Hf), cadmium (Cd), germanium(Ge), chromium (Cr), titanium (Ti), and zinc (Zn). As a specificexample, the oxide semiconductor layer 120 may include: unary metaloxides based on, for example, In—O, Sn—O, Zn—O (ZnO), Ti—O, and Cd—O;binary metal oxides based on, for example, In—Sn—O (ITO), In—Zn—O (IZO),Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, and In—Mg—O; ternary metal oxidesbased on, for example, In—Ga—Zn—O (IGZO), In—Sn—Zn—O (ITZO), In-Al—Zn—O(IAZO), Sn—Ga—Zn—O, Al—Ga—Zn—O, Sn-Al—Zn—O, and Cd—Sn—O; and quaternarymetal oxides based on, for example, In—Sn—Ga—Zn—O. A composition ratioof the above-described oxide semiconductor is not particularly limited,and may be appropriately adjusted within a suitable composition range(e.g., a composition range known in the art). In addition, a thicknessof the oxide semiconductor layer 120 is not particularly limited and maybe appropriately adjusted within a suitable range (e.g., a range knownin the art).

The gate insulating layer 130 is disposed on the oxide semiconductorlayer 120 and serves to insulate the oxide semiconductor layer 120 andthe gate electrode 150 from each other. That is, the gate insulatinglayer 130 may be formed on the substrate 110 to cover an upper portionand a side portion of the oxide semiconductor layer 120.

The oxide semiconductor is a compound semiconductor including ionicbonds of metal cations and oxygen anions. A major carrier of the oxidesemiconductor is an electron, which is n-type. One of the factors thatdetermine the electrical properties of an oxide semiconductor is oxygenvacancy (e.g., oxygen defect). If the oxygen vacancy easily occurs in anoxide semiconductor, it may lead to an increase in concentration of thecarriers in the oxide semiconductor and the mobility may increase.However, when lots of (e.g., excessive amount of) oxygen defects occurdue to oxygen that exists excessively in the oxide semiconductor, thefilm quality of the semiconductor layer is lowered and the stability ofa threshold voltage Vth may be adversely affected. For example, excesscarriers that may be generated by the oxygen defects may cause anincrease in off-current or a change in the threshold voltage Vth.

Particularly, in order to mass-produce panels with oxide semiconductorTFTs, the photoelectric reliability should be secured. In order tosecure the photoelectric reliability, the oxygen deficiency in theactive layer may be controlled by supplying additional oxygen to theoxide semiconductor layer or by performing oxygen treatment on aprotective film, or a subsequent process (such as a high temperatureannealing process) may be performed. These processes may causedisadvantages such as a decrease in a sputtering rate due to theadditional supply of oxygen, an increase in the particle size of theoxide, and/or excessive development of semiconductor characteristicsarising from plasma damage.

Accordingly, in one or more embodiments of the present invention, thestructure of the gate insulating layer 130 that directly contacts theoxide semiconductor layer 120 is partially modified to control thephysical properties of the active layer formed of the oxidesemiconductor, thereby securing the photoelectric reliability.

In an exemplary embodiment, the gate insulating layer 130 that includestwo or more layers is disposed on the oxide semiconductor layer 120. Forexample, a first gate insulating layer 131 that has an oxygen contentlower than that of a stoichiometric composition and/or an excesshydrogen content is disposed at an interface area in contact with theoxide semiconductor layer 120. The hydrogen included in the first gateinsulating layer 131 is diffused to an interface between the first gateinsulating layer 131 and the oxide semiconductor layer 120 and to theinside of the oxide semiconductor layer 120 to absorb excess oxygen.Further, part of the hydrogen introduced into the oxide semiconductorlayer 120 is doped to reduce oxygen defects, thus exhibiting effects ofimproving the photoelectric reliability. In addition, the oxidesemiconductor has characteristics in which the mobility increases as theconcentration of carriers increases to about 10×10²¹ cm⁻³. According toembodiments of the present invention, the hydrogen introduced into theoxide semiconductor layer 120 not only removes interfacial defects, butalso partially increases formation of carriers for transmitting current,and thus electric characteristics such as charge mobility or variationof threshold voltage Vth may be improved.

According to an exemplary embodiment of the present invention, a contentof hydrogen included in the first gate insulating layer 131 may besubstantially equal to or more than about 5.0×10²⁰ atoms/cm³. Forexample, the hydrogen content may be in a range from about 5.0×10²⁰atoms/cm³ to about 2.0×10²¹ atoms/cm³, and more particularly, in a rangefrom about 5.0×10²⁰ atoms/cm³ to about 1.0×10²¹ atoms/cm³. In addition,the first gate insulating layer 131 may have a hydrogen content thatvaries along a thickness direction. For example, the hydrogen contentmay increase toward the center of the first gate insulating layer 131(e.g., along the thickness direction), and may decrease toward oppositeside surfaces of the first gate insulating layer 131.

The oxide semiconductor layer 120 in which hydrogen is partiallyintroduced may have a tendency in which a concentration of oxygendecreases in at least a partial region adjacent to the first gateinsulating layer 131. According to an exemplary embodiment of thepresent invention, comparing to a stoichiometric composition of theoxide semiconductor layer at an interface where the first gateinsulating layer 131 is not provided, the oxygen content of the metaloxide, as compared to a total oxygen content (e.g., 100%) of thecorresponding oxide semiconductor layer 120 according to embodiments ofthe present disclosure, may decrease by about 10% or less. A thicknessof the oxide semiconductor layer in which the oxygen concentration isreduced is not particularly limited.

The oxide semiconductor layer 120 according to an embodiment of thepresent invention in contact with the first gate insulating layer 131may significantly reduce oxygen defects in the oxide semiconductor layer120, as well as reducing the interfacial defects, due to hydrogeninflux. According to another exemplary embodiment of the presentinvention, the oxide semiconductor layer 120 may have a density ofoxygen defects substantially equal to or less than about 2.0×10¹⁷ cm⁻³eV⁻¹, for example, in a range from about 1.50×10¹⁷ cm⁻³ eV⁻¹ to about2.0×10¹⁷ cm⁻³ eV⁻¹.

The first gate insulating layer 131 may include a metal oxide or a metaloxynitride (e.g., that are known in the art) and have an oxygen contentlower than that of the stoichiometric composition. For example, the gateinsulating layer 131 may include at least one material selected from,for example, silicon oxide, aluminum oxide, gallium oxide, titaniumoxide, tantalum oxide, manganese oxide, zinc oxide, zirconium oxide,hafnium oxide, lanthanum oxide, silicon oxynitride, aluminum oxynitride,and gallium oxynitride. As a more specific example, the first gateinsulating layer 131 may include at least one material selected from,for example, SiO_(x) (0<x<2), AlO_(x) (0<x<3/2), ZnO_(x) (0<x<1),ZrO_(x) (0<x<2), HfO_(x) (0<x<2), GaO_(x) (0<x<3/2), TiO_(x) (0<x<2),TaO_(x)(0<x<3/2), MnO_(x) (0<x<2), LaO_(x) (0<x<3/2), SiO_(x)Ny (0<x<1,0<y<2/3, and 0<x+y<5/3), AlO_(x)Ny (0<x<3/4, 0<y<1/2, and 0<x+y<5/4),and GaO_(x)Ny (0<x<3/4, 0<y<1/2, and 0<x+y<5/4).

According to an exemplary embodiment of the present invention, the firstgate insulating layer 131 includes silicon oxide (SiO_(x), where x<2.0),and an atomic weight ratio (O/Si) of oxygen to silicon in the siliconoxide may be substantially equal to or higher than about 1.5 and lessthan about 2.0, for example, in a range from about 1.6 to about 1.8. Inan exemplary embodiment, when the first gate insulating layer 131includes silicon oxide (SiO_(x), where x<2.0), the silicon oxide mayhave an atomic weight ratio (O/Si) that decreases along the thicknessdirection as a distance from the oxide semiconductor layer 120decreases. That is, while the first gate insulating layer 131 based onsilicon oxide has the above-described atomic weight ratio of O/Si, theatomic weight ratio of O/Si of the first gate insulating layer 131 isrelatively high at a region adjacent to the second gate insulating layer132, and is relatively low at a region adjacent to the oxidesemiconductor layer 120. In addition, the thickness of the first gateinsulating layer 131 may be substantially equal to or less than about500 Å, for example, in a range from about 1 Å to about 500 Å. However,embodiments are not limited thereto.

The gate insulating layer 130 may have a multilayer structure includingtwo or more layers, each of which includes a material substantially thesame as or different from the material which the first gate insulatinglayer 131 includes. For example, the gate insulating layer 130 mayinclude: at least one of a second gate insulating layer 132 thatincludes a material substantially the same as the material included inthe first gate insulating layer 131 and has an oxygen content higherthan that of the first gate insulating layer 131; and a third gateinsulating layer 133 that includes a material different from thematerial included in the first gate insulating layer 131. As the firstgate insulating layer 131 described above includes an oxide-basedmaterial or oxynitride-based material, the second gate insulating layer132 may be one of an oxide-based insulating layer and anoxynitride-based insulating layer (e.g., that are known in the art). Inthis embodiment, the second gate insulating layer 132 may have astoichiometric composition that is different from that of the first gateinsulating layer 131 in contact with the second gate insulating layer132. For example, the second gate insulating layer 132 may have anoxide-based or oxynitride-based composition that satisfies astoichiometric ratio. In addition, the third gate insulating layer 133may be one of an oxide-based insulating layer, an oxynitride-basedinsulating layer, and a nitride-based insulating layer. For example, thegate insulating layer 130 may include the first gate insulating layer131 that includes SiO_(x) (where x<2); and the second gate insulatinglayer 132 that includes silicon oxide (SiO₂). In another exemplaryembodiment, the gate insulating layer 130 may include the first gateinsulating layer 131 that includes SiO_(x) (where x<2); the second gateinsulating layer 132 that includes silicon oxide (SiO₂); and the thirdgate insulating layer 133 that includes silicon nitride (SiN).

The thickness of the gate insulating layer 130 is not particularlylimited and may be appropriately adjusted within a suitable range (e.g.,a range known in the art). For example, the thickness of the gateinsulating layer 130 may vary depending on the material of the gateinsulating layer 130, but may be substantially equal to or more thanabout 10 nm and substantially equal to or less than about 10 μm, forexample, in a range from about 50 nm to about 1000 nm, or in a rangefrom about 100 nm to about 500 nm.

The gate electrode 150 is disposed on the gate insulating layer 130, forexample, on the second gate insulating layer 132 or the third gateinsulating layer 133 of the gate insulating layer 130. The gateelectrode 150 may be connected to a gate line for applying an on/offsignal to the TFT. The gate electrode 150 may include a suitable lowresistive metal material (e.g., known in the art). In consideration ofthe closeness of contact to an adjacent layer, surface flatness of alayer to be laminated thereon, and/or processability, the gate electrode150 may have a single-layer structure or a multilayer structure, witheach layer including at least one of, for example, aluminum (Al),platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au),nickel (Ni), neodymium (Nd), iridium (Ir), chromium (Cr), lithium (Li),calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), copper (Cu),and the like.

An insulating interlayer may be disposed on the gate electrode 150. Theinsulating interlayer insulates the gate electrode 150 from the sourceelectrode 140 a and the drain electrode 140 b. The insulating interlayermay have a single-layer structure or a multilayer structure, with eachlayer including an inorganic material. For example, the inorganicmaterial may be metal oxide or metal nitride such as, for example,silicon oxide (SiO₂), silicon nitride (Si₃N₄), silicon oxynitride(SiON), aluminum oxide (Al₂O₃), titanium oxide (TiO₂), tantalum oxide(Ta₂O₅), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), or the like.

The source electrode 140 a and the drain electrode 140 b are disposed onthe gate insulating layer 130, and may be disposed on the insulatinginterlayer in a selective manner. Each of the source electrode 140 a andthe drain electrode 140 b may have a single-layer structure or amultilayer structure, with each layer including a suitable metal (e.g.,a metal known in the art). The metal may be at least one of, forexample, aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag),magnesium (Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir),chromium (Cr), lithium (Li), calcium (Ca), molybdenum (Mo), titanium(Ti), tungsten (W), copper (Cu), and the like. The source electrode 140a and the drain electrode 140 b are each disposed to contact an area ofthe oxide semiconductor layer 120. For example, the gate insulatinglayer 130 and/or the insulating interlayer is formed to expose a sourcearea and a drain area of the oxide semiconductor layer 120, and thesource electrode 140 a and the drain electrode 140 b are formed tocontact the source area and the drain area of the oxide semiconductorlayer 120, respectively.

FIG. 2 is a cross-sectional view illustrating a TFT substrate 200according to another embodiment of the present invention. In FIG. 2, thesame reference numerals as those in FIG. 1 denote the same members.

In the following description of FIG. 2, the descriptions provided abovewith reference to FIG. 1 will not be given again, and only differenceswill be described. Referring to FIG. 2, the TFT substrate 200 accordingto the present embodiment includes an insulating layer 130 that has atriple-layer structure in which a first gate insulating layer 131, asecond gate insulating layer 132, and a third gate insulating layer 133are sequentially stacked, as compared with FIG. 1. Here, the first gateinsulating layer 131 in the gate insulating layer 130 contacts an oxidesemiconductor layer 120, and part of the third gate insulating layer 133contacts a gate electrode 150.

The description of the TFT of FIG. 1 can be applied to the material, thestructure, the oxide semiconductor, and the like of each constituentelement of the present embodiment as it is.

FIG. 2 specifically illustrates an exemplary embodiment in which thefirst gate insulating layer 131, the second gate insulating layer 132,and the third gate insulating layer 133 are sequentially disposed as thegate insulating layer 130. However, embodiments are not limited thereto,and the stack structure of the gate insulating layer 130 and the numberof insulating layers of the gate insulating layer 130 are notparticularly limited, provided that the first gate insulating layer 130that has an oxygen content lower than that of the stoichiometriccomposition exists in an area of the gate insulating layer 130 incontact with the oxide semiconductor layer 120. That is, the gateinsulating layer 130 may further include a plurality of insulatinglayers, and the number of times of stacking the insulating layers is notparticularly limited.

FIG. 3 is a cross-sectional view illustrating a TFT substrate 300according to another embodiment of the present invention. In FIG. 3, thesame reference numerals as those in FIG. 1 denote the same members.

In the following description with reference to FIG. 3, the descriptionsprovided above with reference to FIG. 1 will not be given again, andonly differences will be described. Unlike the exemplary embodiments ofFIGS. 1 and 2, which illustrate the top gate structure, the TFT of FIG.3 has a bottom gate structure in which a gate electrode 150 is locatedbelow an oxide semiconductor layer 120. In one embodiment, the TFTsubstrate 300 includes a substrate 110; the gate electrode 150 disposedon the substrate 110; a gate insulating layers 130 disposed on the gateelectrode 150 and including at least two layers that includes a firstgate insulating layer 131; the oxide semiconductor layer 120 disposed onthe gate insulating layer 130; and a source electrode 140 a and a drainelectrode 140 b connected to the oxide semiconductor layer 120 andspaced apart from each other.

The description of the TFT of FIG. 1 can be applied to the material, theoxide semiconductor, and the like of each constituent element of anexemplary embodiment of FIG. 3, and thus will not be repeated.

FIGS. 1 to 3 illustrate TFTs having a top gate and a bottom gatestructure as a specific example. However, embodiments are not limitedthereto, and various kinds of TFTs, such as a coplanar type, a staggeredtype, an inverted staggered type, an etch stop layer (ESL) type, may bealso applied and are also included in the scope of the presentinvention.

FIGS. 4 to 8 are cross-sectional views schematically illustrating amethod of manufacturing the TFT substrate 100 of FIG. 1.

In the present disclosure, a method of manufacturing the above-describedtop gate TFT substrate 100 will be described by way of example. However,the method of manufacturing the TFT related to the present embodimentcan be similarly applied to the case of manufacturing another kind ofTFT substrates.

As illustrated in FIG. 4, the substrate 110 for forming a TFT isprepared, and then the oxide semiconductor layer 120 is formed on asurface of the substrate 110.

The substrate 110 may be provided in the form of a planar plate, and maybe any one selected from a glass substrate, a plastic substrate, a metalsubstrate, and the like.

The oxide semiconductor layer 120 may have the structure illustrated inFIG. 1, and various suitable metal oxides (e.g., metal oxides known inthe art) may be applied. As an example, the oxide semiconductor layer120 may be formed utilizing a suitable one of wet process methods,physical methods, and/or chemical methods (e.g., a method known in theart). In one embodiment, the oxide semiconductor layer 120 may beformed, in a selective manner, by wet process methods (such as printingand coating); physical methods (such as vacuum deposition, sputteringand ion plating); and/or chemical methods (such as CVD and plasma CVD).In one embodiment, a vapor deposition method, such as a vacuum vapordeposition method, a sputtering method, an ion plating method, a CVDmethod, and/or a plasma CVD method, is utilized in consideration of thecontrollability of thickness of the film.

In the present disclosure, the buffer layer may be formed on an uppersurface of the substrate 110 before forming the oxide semiconductorlayer 120, in order to substantially prevent penetration of moisture oroutside air, and to planarize the surface. The buffer layer may beformed by various deposition methods such as plasma enhanced chemicalvapor deposition (PECVD), atmospheric pressure CVD (APCVD), and/or lowpressure CVD (LPCVD).

After the oxide semiconductor layer 120 is formed, a patterning processmay be performed to form a set or predetermined shape byphotolithography, etching, lift-off, and/or the like, if necessary. Anannealing process may be performed to adjust the electrical resistivityand the like.

Then, the first gate insulating layer 131 is formed on the oxidesemiconductor layer 120 as illustrated in FIG. 5.

The first gate insulating layer 131 may be formed utilizing a suitabledeposition method (e.g., a method known in the art). For example, thefirst gate insulating layer 131 may be formed utilizing one of methodsincluding, for example, plasma enhanced CVD (PECVD), plasma enhancedatomic layer deposition (PEALD), atomic layer deposition (ALD),metal-organic CVD (MOCVD), thermal CVD, and/or sputtering. In oneembodiment, the first gate insulating layer 131 is formed utilizing aPECVD method.

According to an embodiment of the present invention, the first gateinsulating layer 131 is formed utilizing plasma. In one embodiment, thefirst gate insulating layer 131 is formed utilizing a power P1, a gascomposition (also referred to as a first gas hereinafter) G1, or both ofthe power P1 and the gas composition G1, where the power P1 and the gascomposition G1 utilized to form the first gate insulating layer 131 arelower than a power P2 and a gas composition (also referred to as asecond gas hereinafter) G2, respectively, that are required to form thesecond gate insulating layer (e.g., SiO₂) 132 having a stoichiometriccomposition.

As an example, the first gate insulating layer 131 may be formed byapplying the first power P1 that is lower than the second power P2applied to form the second gate insulating layer 132. In one embodiment,when the second power P2 required for forming the second gate insulatinglayer (e.g., SiO₂) 132 that satisfies the stoichiometric composition is10 kW, when the first power P1 that is lower than the first power P1 isapplied, an insulating layer having an incomplete (e.g., unbalanced)chemical composition is formed. Such an insulating layer having anincomplete chemical composition has an oxygen content lower than that ofthe stoichiometric composition and/or includes excess hydrogen. Thefirst power P1 for forming the first gate insulating layer 131 is notparticularly limited as long as it is lower than the second power P2.For example, the first power P1 may be substantially equal to or lowerthan about 80% of the second power P2. As an example, when the secondpower P2 is 10 kW, the first power P1 may be about 8 kW or less, forexample, in a range from about 1 kW to about 8 kW.

As another example, the first gate insulating layer 131 may be formedutilizing the first gas G1 having a composition ratio (flow ratio) lowerthan that of the second gas G2 for forming the second gate insulatinglayer 132. For example, when the first gas G1 having a composition ratio(flow ratio) lower than that of the second gas G2 required to form thesecond gate insulating layer (e.g., SiO₂) 132 that satisfies thestoichiometric composition, an insulating layer having an incompletechemical composition and including excess hydrogen is formed as theabove exemplary embodiment. The composition of the first gas G1 forforming the first gate insulating layer 131 is not particularly limited,and may be lower than the composition of the second gas G2 for formingthe second gate insulating layer 132. For example, the first gas G1 mayhave a gas composition substantially equal to or more than about 1 andless than about 50. As used herein, each composition of the first gas G1and the second gas G2 refers to a volume ratio between a reaction gasand a metal precursor gas (e.g., reaction gas/metal precursor gas). Forexample, the reaction gas may be N₂O, O₃, O₂, NH₃, H₂, N₂, or a mixturethereof. The metal precursor gas may be a precursor gas including atleast one element selected from, for example, Si, Al, Ti, Ga, Ta, Mn,Zn, Zr, Hf and La. In one embodiment, silane (SiH₄), tetrakis(dimethylamino) titanium (TDMAT), trimethyl aluminum (TMA), and/or thelike may be utilized as the metal precursor gas. The metal precursor gasmay further include an inert gas.

In an exemplary embodiment, the gas composition (flow ratio) variesdepending on the component of an insulating layer to be formed. Forexample, when a silicon oxide (SiO₂) layer that satisfies thestoichiometric composition is formed, the volume ratio between thereactive gas and the metal precursor gas (e.g., N₂O:SiH₄) is about100:2, and thus the gas composition (reaction gas/metal precursor gas)forming the second gas G2 may be about 50. Accordingly, when the firstgate insulating layer (SiO_(2-x)) 131 having an oxygen content lowerthan that of the stoichiometric composition is formed, the volume ratiobetween the reactive gas and the metal precursor gas (e.g., N₂O:SiH₄)may be adjusted to be less than about 100:2 and substantially equal toor greater than about 100:100, and thus the gas composition (e.g., avolume ratio of reaction gas/metal precursor gas) of the first gas G1may be substantially equal to or more than about 1 and less than about50.

In another exemplary embodiment, the first gate insulating layer 131 maybe formed utilizing both the first power P1 and the first gas G1.

Then, the second gate insulating layer 132 is formed on the first gateinsulating layer 131, as illustrated in FIG. 6.

The second gate insulating layer 132 may be formed utilizing the samemethod utilized to form the first gate insulating layer 131 describedabove, except that at least one of the second power P2 and the secondgas G2 that are higher than the first power P1 and the first gas G1,respectively, is utilized. The second gate insulating layer 132 mayinclude a material substantially the same as a material included in thefirst gate insulating layer 131, but may have a stoichiometriccomposition different from that of the first gate insulating layer 131.In one embodiment, the second gate insulating layer 132 may be amaterial that satisfies the stoichiometric ratio.

Although not illustrated in the drawing, a process of forming the thirdgate insulating layer 133 on the second gate insulating layer 132 may befurther performed. The third gate insulating layer 133 may include amaterial different from the material included in the first gateinsulating layer 131 and the second gate insulating layer 132. Inaddition, the third gate insulating layer 133 may have a multilayerstructure including two or more layers, with each layer includingdifferent materials.

FIG. 9 is a graph illustrating plasma power utilized for manufacturing afirst gate insulating layer GI1 and a second gate insulating layer GI2.Referring to FIG. 9, the first gate insulating layer GI1 is formedthrough plasma treatment with a first power (kW) P1, and the second gateinsulating layer GI2 is formed through plasma treatment with a secondpower (kW) P2 that is higher than the first power (kW) P1. For example,the first power (kW) P1 may be substantially equal to or less than about80% of the second power (kW) P2. For example, when the second power (kW)P2 is 10 kW, the first power (kW) P1 may be substantially equal to orless than about 8 kW.

FIG. 10 is a graph illustrating gas composition ratios utilized formanufacturing the first gate insulating layer GI1 and the second gateinsulating layer GI2. Referring to FIG. 10, the first gate insulatinglayer GI1 is formed through plasma treatment utilizing the first gas G1,and the second gate insulating layer GI2 is formed through plasmatreatment utilizing the second gas G2 that has a composition ratio (flowratio) higher than that of the first gas G1. For example, the first gasG1 may have a composition ratio (e.g., a volume ratio of reactiongas/metal precursor gas) lower than that of the second gas G2, and maybe, for example, substantially equal to or more than about 1 and lessthan about 50.

An annealing process may be further performed after forming the gateinsulating layer 130 including at least two layers. The temperaturerange of the annealing process is not particularly limited and may beappropriately adjusted within a suitable range (e.g., a range known inthe art). When such an annealing process is performed after the gateinsulating layer 130 is formed, part of the hydrogen included in thefirst gate insulating layer 131 is diffused to the interface between thefirst gate insulating layer 131 and the oxide semiconductor layer 120and/or to the inside of the oxide semiconductor layer 120. The hydrogenintroduced to the oxide semiconductor may absorb excess oxygen, thusreducing a density of oxygen defects in a sub-gap of a conduction bandof the oxide semiconductor.

After the gate insulating layer 130 is formed, the gate electrode 150 isformed as illustrated in FIG. 7.

The gate electrode 150 may be formed by a suitable method (e.g., amethod known in the art). For example, a conductive film is formed bywet process methods such as printing and coating; physical methods suchas vacuum deposition, sputtering and ion plating; and/or chemicalmethods such as CVD and plasma CVD, in consideration of suitability witha material. After the conductive film is formed, the conductive film ispatterned into a set or predetermined shape by photolithography,etching, lift-off, and/or the like, and thus the gate electrode 150 isformed from the conductive film. In this embodiment, the gate electrode150 and gate wirings may be patterned concurrently or substantiallysimultaneously. By patterning the gate electrode 150, the gateinsulating layer 130 may be exposed to the outside through the portionon an outer circumferential surface of the gate electrode 150.

The conductive film forming the gate electrode 150 may include anysuitable material having high conductivity. For example, the conductivefilm forming the gate electrode 150 may include a metal and/or a metaloxide conductive layer that includes at least one of: metals selectedfrom Al, Mo, Cr, Ta, Ti and Au; AI-Nd; Ag alloys; tin oxide; zinc oxide;indium oxide; indium tin oxide (ITO); indium zinc oxide (IZO); and thelike.

Next, the source electrode 140 a and the drain electrode 140 b areformed, as illustrated in FIG. 8, and are each connected to the oxidesemiconductor layer 120.

The source electrode 140 a and the drain electrode 140 b may each beformed utilizing a suitable method (e.g., a method known in the art),and then connected to the oxide semiconductor layer 120. For example,after forming an insulating interlayer over an entire surface of thesubstrate 110 including the gate electrode 150, the gate insulatinglayer 130 and the insulating interlayer are etched to define a contacthole, a low resistance metal layer is deposited thereon and etched, andthus each of the source electrode 140 a and the drain electrode 140 bthat contacts the oxide semiconductor layer 120 is formed.

In the present disclosure, a method of manufacturing a top gate TFTsubstrate in which the oxide semiconductor layer 120, the gateinsulating layer 130 that includes at least two layers including thefirst gate insulating layer 131, the gate electrode 150, the sourceelectrode 140 a and the drain electrode 140 b are stacked is describedby way of example. However, embodiments are not limited to the abovemanufacturing method, and each process may be modified or optionallymixed as necessary.

In the present disclosure, the oxygen content and/or the amount ofhydrogen included in the first gate insulating layer is changedaccording to the plasma power P1 and/or the gas composition ratio (flowratio) G1 utilized to form the film. The amount of hydrogen can beaccurately analyzed utilizing a hydrogen concentration analysis methodsuch as secondary ion mass spectroscopy (SIMS).

FIG. 11 is a secondary ion mass spectroscopy (“SIMS”) graph illustratingchanges in the content of hydrogen in the first gate insulating layer(SiO_(x), where x<2) according to the plasma power. Referring to FIG.11, it may be appreciated that the hydrogen content in the first gateinsulating layer significantly increases as the plasma power P1 islowered.

FIG. 12 is a SIMS graph illustrating changes in the content of hydrogenincluded in a gate insulating layer according to the sub-layers.Referring to FIG. 12, it may be appreciated that the hydrogen content ofthe first gate insulating layer GI1 is relatively higher than thehydrogen content of the second gate insulating layer GI2 according to anembodiment of the present disclosure (i.e., sample identified as GI1, incontrast to the sample identified as Ref). The hydrogen content of thefirst gate insulating layer GI1 varies along the thickness directionthereof. There appears the tendency that the hydrogen content of thefirst gate insulating layer GI1 increases toward the center of the firstgate insulating layer GI1, and decreases toward opposite side surfacesof the first gate insulating layer GI1 (e.g., towards GI2 and towardsactive). In this embodiment, because the third gate insulating layer GI3includes a SiN_(x) layer, it may be appreciated that the hydrogencontent thereof is the highest in the gate insulating layer.

FIG. 13 is a graph illustrating an O/Si atomic weight ratio in the gateinsulating layer based on silicon oxide. Referring to FIG. 13, Refrepresents a gate insulating layer that does not include the first gateinsulating layer, which exhibits an O/Si atomic weight ratio of about 2over the entire region (e.g., the entire gate insulating layer), thussubstantially satisfying a stoichiometric composition (SiO₂). On theother hand, an O/Si atomic weight ratio of the first gate insulatinglayer GI1 is about 1.69, which is outside (i.e., below) thestoichiometric ratio. Particularly, the O/Si atomic weight ratio of thefirst gate insulating layer GI1 decreases as a distance from the oxidesemiconductor layer decreases. This tendency may exhibit substantiallythe same for hydrogen concentration distribution even when hydrogen gasis utilized in comparable vapor deposition methods (e.g., MOCVD and ALD,methods known in the art), other than plasma treatment (e.g., PECVD).

FIGS. 14 to 16 are graphs illustrating changes in density of state(“DOS”) of the oxide semiconductor layer.

Excess oxygen, which is in an electrically neutral state, is generallyweakly bonded to oxygen atoms in the oxide semiconductor and is presentas an O—O dimer. Such an O—O dimer has an electronic structure similarto that of an oxygen molecule, and thus causes an increase in DOS in thevicinity of a lower end of the conduction band of the oxidesemiconductor, that is, in the sub-gap region.

FIG. 14 shows the conduction band of the oxide semiconductor layer incontact with the first gate insulating layer. Referring to FIG. 14,because the oxide semiconductor layer of the present disclosure is incontact with the first gate insulating layer, DOS in the sub-gap of theconduction band is decreased.

FIGS. 15 and 16 show respective changes in DOS of an oxide semiconductorlayer Ref that does not include the first gate insulating layer, anoxide semiconductor layer GI1-1 in contact with a first gate insulatinglayer formed by controlling the plasma power, and an oxide semiconductorlayer GI1-2 in contact with a first gate insulating layer formed bycontrolling the gas composition ratio (flow ratio).

Referring to FIG. 16, the oxide semiconductor layer Ref that does notinclude the first gate insulating layer shows a density of oxygendefects of about 2.27E+17 cm⁻³ eV⁻¹. On the other hand, the oxidesemiconductor layer GI1-1 in contact with the first gate insulatinglayer formed by controlling the plasma power shows a density of oxygendefects of about 1.64E+17 cm⁻³ eV⁻¹, and the oxide semiconductor layerGI1-2 in contact with the first gate insulating layer formed bycontrolling the gas composition ratio shows a density of oxygen defectsof about 1.72E+17 cm⁻³ eV⁻¹, indicating that the oxygen defects weresignificantly reduced.

Based on the results shown in FIGS. 11 to 16, it may be appreciated thatthe first gate insulating layer according to the present disclosureincludes excess hydrogen, which flows into the adjacent oxidesemiconductor to reduce excess oxygen and provides effects of defectstate passivation.

FIGS. 17 and 18 are graphs illustrating positive bias temperature stress(“PBTS”) values of TFTs.

Referring to FIG. 17, when about 1 nA is applied for about 1 hour underconditions of a gate voltage Vgs of about 37 V, a drain voltage Vds ofabout 0 V, and a temperature of about 70° C., a TFT that does notinclude the first gate insulating layer shows a shift of a thresholdvoltage ΔVth by about 2.92 V according to the stress time. On the otherhand, a TFT that includes the first gate insulating layer illustrated inFIG. 18 shows a shift of a threshold voltage ΔVth by about 0.21 Vaccording to the stress time under the same condition, indicating thedesired effect in terms of stability of the threshold voltage Vth.

FIG. 19 is a graph illustrating changes in a threshold voltage Vth withrespect to a cumulative time based on the results of FIGS. 17 and 18.Referring to FIG. 19, it may be appreciated that the TFT of the presentdisclosure that includes the first gate insulating layer secures thestability of the threshold voltage Vth (e.g., improved by about 100times or more) as compared with that of the TFT that does not includethe first gate insulating layer.

According to the above-described embodiments, as the TFT substrate ofthe present disclosure includes the first gate insulating layer 131having an oxygen content lower than that of the stoichiometriccomposition and/or including excess hydrogen on the interface of thegate insulating layer 130 in contact with the oxide semiconductor layer120, the defects due to excess oxygen in the oxide semiconductor layer120 may be controlled and the photoelectric reliability of the elementmay be improved.

The TFT substrate including the first gate insulating layer according toembodiments of the present invention and the modified exemplaryembodiments thereof may be applied without limitation to display devicesthat are known in the art and include TFTs.

In the present disclosure, the display device refers to a device fordisplaying images, and is not limited to flat panel display devices(FPDs). Non-limiting examples of the display device may include curveddisplay devices, foldable display devices, and flexible display devices.Non-limiting examples of the display device may be liquid crystaldisplay (LCD) devices, electrophoretic display devices, organic lightemitting diode (“OLED”) display devices, inorganic light emittingdisplay devices, field emission display devices, surface-conductionelectron-emitter display devices, plasma display devices, cathode raydisplay devices, and/or the like.

As set forth hereinabove, one or more embodiments of the presentinvention may provide a TFT that has high reliability by controllingdefects due to excess oxygen included in the oxide semiconductor.Accordingly, a display device including the oxide semiconductor TFTsthat have high reliability may be manufactured.

The effects of embodiments of the present invention are not limited bythe descriptions exemplified above, and more various effects may bepresented.

While the present invention has been illustrated and described withreference to the embodiments thereof, it will be apparent to those ofordinary skill in the art that various changes in form and detail may beformed thereto without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A thin film transistor substrate comprising: asubstrate; an oxide semiconductor layer on the substrate; a gateelectrode on the substrate; a gate insulating layer between the oxidesemiconductor layer and the gate electrode; and a source electrode and adrain electrode both connected to the oxide semiconductor layer, thesource electrode and the drain electrode being spaced apart from eachother, wherein the gate insulating layer comprises: a first gateinsulating layer comprising a first material and having an oxygencontent lower than that of a stoichiometric composition of the firstmaterial; and a second gate insulating layer comprising a secondmaterial substantially the same as the first material, and having anoxygen content higher than that of the first gate insulating layer, andthe first gate insulating layer and the oxide semiconductor layerdirectly contact each other.
 2. The thin film transistor substrate ofclaim 1, wherein the first gate insulating layer comprises a metal oxideor a metal oxynitride that has an oxygen content lower than that of thestoichiometric composition.
 3. The thin film transistor substrate ofclaim 1, wherein the first gate insulating layer comprises one or morematerials selected from: SiO_(x) (0<x<2), AlO_(x)(0<x<3/2), ZnO_(x)(0<x<l), ZrO_(x) (0<x<2), HfO_(x) (0<x<2), GaO_(x) (0<x<3/2), TiO_(x)(0<x<2), TaO_(x) (0<x<3/2), MnO_(x) (0<x<2), LaO_(x) (0<x<3/2),SiO_(x)Ny (0<x<1, 0<y<2/3, and 0<x+y<5/3), AlO_(x)Ny (0<x<3/4, 0<y<1/2,and 0<x+y<5/4), and GaO_(x)Ny (0<x<3/4, 0<y<1/2, and 0<x+y<5/4).
 4. Thethin film transistor substrate of claim 1, wherein the first gateinsulating layer comprises silicon oxide represented by SiO_(x), where0<x<2, and an atomic weight ratio O/Si of oxygen to silicon in thesilicon oxide is substantially equal to or more than about 1.5 and lessthan about 2.0.
 5. The thin film transistor substrate of claim 4,wherein the atomic weight ratio O/Si in the silicon oxide decreasesalong a thickness direction as a distance from the oxide semiconductorlayer decreases.
 6. The thin film transistor substrate of claim 1,wherein the first gate insulating layer has an excess hydrogen contentof about 5.0×10²⁰ atoms/cm³.
 7. The thin film transistor substrate ofclaim 1, wherein the first gate insulating layer has a hydrogen contentthat varies along a thickness direction of the first gate insulatinglayer.
 8. The thin film transistor substrate of claim 1, wherein thefirst gate insulating layer has a thickness substantially equal to orless than about 500 Å.
 9. The thin film transistor substrate of claim 1,wherein the gate insulating layer further comprises a third gateinsulating layer comprising a third material different from the firstmaterial.
 10. The thin film transistor substrate of claim 9, wherein thethird gate insulating layer is an oxide-based insulating layer, anoxynitride-based insulating layer, or a nitride-based insulating layer.11. The thin film transistor substrate of claim 1, wherein the oxidesemiconductor layer comprises a metal oxide semiconductor comprisingoxygen, and has a density of oxygen defects of about 2.0×10¹⁷ cm⁻³ eV⁻¹.12. The thin film transistor substrate of claim 1, wherein the oxidesemiconductor layer comprises at least one selected from: indium (In),gallium (Ga), tin (Sn), zirconium (Zr), vanadium (V), hafnium (Hf),cadmium (Cd), germanium (Ge), chromium (Cr), titanium (Ti), and zinc(Zn).
 13. The thin film transistor substrate of claim 1, comprising: thesubstrate; the oxide semiconductor layer on the substrate; the firstgate insulating layer on the oxide semiconductor layer; the second gateinsulating layer on the first gate insulating layer; the gate electrodeon the second gate insulating layer; and the source electrode and thedrain electrode both connected to the oxide semiconductor layer, thesource electrode and the drain electrode being spaced apart from eachother.
 14. The thin film transistor substrate of claim 1, comprising:the substrate; the gate electrode on the substrate; the second gateinsulating layer on the gate electrode; the first gate insulating layeron the second gate insulating layer; the oxide semiconductor layer onthe first gate insulating layer; and the source electrode and the drainelectrode both connected to the oxide semiconductor layer, the sourceelectrode and the drain electrode being spaced apart from each other.